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From "Stu Hood (JIRA)" <>
Subject [jira] Updated: (CASSANDRA-2156) Compaction Throttling
Date Sat, 12 Feb 2011 03:12:57 GMT


Stu Hood updated CASSANDRA-2156:

    Attachment:     (was: for-0.6-0001-Throttle-compaction-to-a-fixed-throughput.txt)

> Compaction Throttling
> ---------------------
>                 Key: CASSANDRA-2156
>                 URL:
>             Project: Cassandra
>          Issue Type: New Feature
>            Reporter: Stu Hood
>             Fix For: 0.8
>         Attachments: for-0.6-0001-Throttle-compaction-to-a-fixed-throughput.txt, for-0.6-0002-Make-compaction-throttling-configurable.txt
> Compaction is currently relatively bursty: we compact as fast as we can, and then we
wait for the next compaction to be possible ("hurry up and wait").
> Instead, to properly amortize compaction, you'd like to compact exactly as fast as you
need to to keep the sstable count under control.
> For every new level of compaction, you need to increase the rate that you compact at:
a rule of thumb that we're testing on our clusters is to determine the maximum number of buckets
a node can support (aka, if the 15th bucket holds 750 GB, we're not going to have more than
15 buckets), and then multiply the flush throughput by the number of buckets to get a minimum
compaction throughput to maintain your sstable count.
> Full explanation: for a min compaction threshold of {{T}}, the bucket at level {{N}}
can contain {{SsubN = T^N}} 'units' (unit == memtable's worth of data on disk). Every time
a new unit is added, it has a {{1/SsubN}} chance of causing the bucket at level N to fill.
If the bucket at level N fills, it causes {{SsubN}} units to be compacted. So, for each active
level in your system you have {{SubN * 1 / SsubN}}, or {{1}} amortized unit to compact any
time a new unit is added.

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